Hubble Space Telescope Probes the Compact Nucleus of Galaxy M87
16 January 1992Astronomers report today that they have found intriguing evidence that a black hole weighing over 2.6 billion times the mass of the Sun exists at the center of the giant elliptical galaxy M87, based upon images taken by NASA's Hubble Space Telescope (HST). The images show that stars become strongly concentrated towards the center of M87, as if drawn into the center and held there by the gravitational field of a massive black hole.
These results are being reported at the 179th meeting of the American Astronomical Society in Atlanta, Georgia by Dr. Tod R. Lauer, of the National Optical Astronomy Observatories, Dr. Sandra M. Faber of the University of California, Santa Cruz; Dr. C. Roger Lynds, also of NOAO, and other members of the HST Wide Field/Planetary Camera (WFPC) Imaging Team.
M87 is at the center of a nearby cluster of galaxies in the constellation of Virgo, 52 million light- years distant, and contains more than 100 billion stars. One of the brightest galaxies in the local universe, M87 is visible in even small telescopes.
Early in this century astronomers discovered a gigantic plume or "jet" of plasma apparently ejected out of the M87 nucleus. Later, the jet and nucleus were found to emit strong radio and X- ray radiation. However, the nature of the central "engine" of this activity has long remained a mystery. In 1978, the late Peter Young, of the California Institute of Technology, leading a team of astronomers, announced that the central portions of M87 visible from the ground appeared to be dominated by the gravity of a massive black hole. However, prior to the HST observations, more recent ground-based observational and theoretical studies have failed to confirm this picture.
Lauer, Faber, Lynds and co-investigators on the WFPC imaging team used the new images obtained with the HST Planetary Camera to explore the central structure of M87 much closer into its nucleus than is possible from the ground. The images show clearly that the stars in M87 become densely concentrated towards the center, forming a bright "cusp" of light at the heart of the galaxy.
The central density of stars in M87 is at least 300 times greater than expected for a normal giant elliptical galaxy, and over a thousand times denser than the distribution of stars in the neighborhood of our own Sun. In fact, the ultimate central density of stars in M87 may be even higher, but its measurement is beyond the resolving power of even HST.
"The central structure of M87 is a striking departure from what the normal core of a giant elliptical galaxy would look like," says Lauer. He adds, "It strongly resembles a stellar cusp associated with a black hole." The cusp is visible as the steady increase in brightness of M87 toward its center. Theoretical work suggests that such a cusp may form as a central black hole grows and causes the center of the galaxy to collapse outwards.
Early in the life of M87 a "seed" black hole may have formed in its nucleus from the merger of small black holes created by the explosion of massive stars, or perhaps from the gravitational collapse of gas leftover from the formation of M87. Once formed, the seed black hole would grow by feeding on gas and stars that passed by too closely. As the mass of the black hole increased, its gravity would begin to dominate an increasingly larger volume of space. Stars once freely orbiting in and out of the M87 core would be gradually pulled towards the center and then into orbits closely bound to the black hole. The whole core of the galaxy thus smoothly collapses inward, and the density of stars near the very center becomes extreme. Some of these stars may be eventually consumed by the black hole, fueling its growth further. This leads to an interesting paradox that one way to look for a black hole is to search for a strong concentration of starlight at the center of a galaxy.
The mass of the black hole is estimated at 2.6 billion times that of the Sun based on comparing the density of stars in the cusp to theoretical models computed by Peter Young a decade ago. Lauer emphasizes, however, that the HST images alone don't prove conclusively the black hole's presence. "It looks like a 'duck' but we haven't heard it 'quack' yet," he observed. Follow-on HST spectroscopic observations are needed to measure the velocity of stars orbiting within the nucleus. High velocities would be evidence of a black hole and would provide astronomers with direct measurement of its mass.
The search for super massive black holes in the cores of galaxies is one of the primary missions of NASA's Hubble Space Telescope. By investigating both active and quiescent galaxies astronomers will have a better idea of the conditions and events which lead to the formation and growth of super-massive black holes.